The present invention relates to a radar system, such as an FMCW (Frequency Modulated Continuous Wave) radar, which is preferably used for preventing a collision of a movable object. In such a radar system, a radar wave carrying a frequency-modulated transmitting signal is emitted toward a target object. Then, the radar wave is reflected from the target object and received by the radar system. During such transmitting and receiving operations of the radar wave, obtained information is used for calculating a relative distance and/or a relative velocity of the target object.
To enhance the safety of an automotive vehicle, installing a radar system into an automotive vehicle is a prospective attempt. The FMCW radar is suitable for the use in an automotive vehicle because of its capability of simultaneously detecting a distance and a relative velocity of an object as well as simple, compact and cheap structural features.
FIG. 30A shows frequency modulations of a transmitting signal Ss and a reflected received signal Sr of a conventional FMCW radar. As shown by a solid line, the transmitting signal Ss is frequency modulated by a triangular wave modulation signal, so that the frequency increases linearly relative to time and then decreases reversely. A radar wave carrying the transmitting signal Ss is emitted through a transmitter antenna toward a target object. The radar wave is then reflected from the target object and received by a receiver antenna of the radar system. A dotted line shows the received signal Sr.
There is a significant delay time Td between the transmitting signal Ss and the received signal Sr. This delay time Td is a time required for the radar wave going and returning between the radar system and the target object. In other words, the delay tie Td is proportional to the distance between the radar system and the target object.
Furthermore, there is a significant frequency difference Fd between the transmitting signal Ss and the received signal Sr. This frequency difference Fd is a Doppler shift factor caused by a relative velocity between the radar system and the target object.
The received signal Sr and the transmitting signal Ss are mixed in a mixer. FIG. 30B shows a beat signal Sb produced from the mixer. The beat signal Sb represents a frequency difference between the transmitting signal Ss and the received signal Sr.
The beat signal Sb takes different values in accordance with the type of frequency modulation. When the transmitting signal Ss has an increasing frequency, the beat signal Sb has a frequency referred to as an ascendant modulation frequency fu. When the transmitting signal Ss has a decreasing frequency, the frequency of the beat signal Sb is referred to as a descendant modulation frequency fd.
The following equations (1) and (2) define a distance R and a relative velocity V of the target object. ##EQU1## where "c" represents a radio wave propagation speed, "T" represents a cyclic period of a triangular wave used for modulating the transmitting signal Ss, .DELTA.F represents a frequency variation width of the transmitting signal Ss, and Fo represents a central frequency of the transmitting signal Ss (refer to FIG. 30A).
When the FMCW radar system is installed in an automotive vehicle, it is usually necessary to have a detectable distance in a range of approximately 100 to 200 m with a detecting ability equivalent to a resolution level of several meters.
A distance resolution .DELTA.R of the FMCW radar system is generally expressed by the following equation (3). ##EQU2##
As apparent from the equation (3), the distance resolution .DELTA.R of several meters is obtained when the frequency variation width .DELTA.F of the transmitting signal Ss is approximately 100 MHz. To obtain such a desirable frequency variation width .DELTA.F, the central frequency Fo of the transmitting signal Ss needs to be set in a frequency range of several tens to hundreds GHz, which is generally referred to as "millimeter wave."
For example, the transmitting signal Ss may have the frequency variation width .DELTA.F=100 MHz and the cyclic period T=1 ms. The target object may have the relative velocity V=0 (i.e., fu=fd) and be located at the distance R=100 m. In such a condition, the beat frequencies fu and fd would be 133 kHz.
When the target object is located within 100 m, the detectable beat signal Sb is not larger than 133 kHz. When relative velocity V is not zero, a frequency of the detectable beat signal Sb differs from the frequency corresponding to V=0 by a Doppler shift amount. In short, when installed in an automotive vehicle, the radar system must have a capability of detecting the beat signal in a range of several tens to hundreds kHz.
However, in processing millimeter waves, a high-frequency mixer is subjected to AM-FM conversion noises composed of frequency components of signal intensity fluctuation or 1/f noises having intensities inverse proportional to the frequencies. The AM-FM conversion noises and the 1/f noises are collectively referred to as low-frequency noises whose intensities are relatively strong in a frequency range of several tens to hundreds kHz of the beat signal Sb. This leads to a serious deterioration in a signal versus noise ratio (hereinafter referred to S/N ratio) of the beat signal Sb.
FIG. 31 shows a conventional FMCW radar system disclosed in Published Unexamined Japanese Patent Application No. 5-40169 whose counterpart U.S. patent application is patented as U.S. Pat. No. 5,381,153. According to this radar system 110, a high-frequency oscillator 112 produces a high-frequency transmitting signal Ss. A modulation signal generating circuit 126 generates a modulation signal Sm. The transmitting signal Ss produced from the high-frequency oscillator 112 is modulated in accordance with this modulation signal Sm. Through this modulation, the frequency of the transmitting signal Ss causes a triangular change.
A transmitter antenna 16 emits a radar wave carrying the transmitting signal Ss supplied from the high-frequency oscillator 112. The radar wave is reflected from a target object and received by a receiver antenna 120. The receiver antenna 120 sends a received signal Sr to a high-frequency mixer 122. A distributor 118 separates part of the transmitting signal Ss and sends it as a local signal L to the high-frequency mixer 122. The high-frequency mixer 122 mixes the received signal Sr with the local signal L, and produces a beat signal Sb.
A second oscillator 136 produces a switching signal whose frequency is larger than two times the beat signal Sb. A switching circuit 138 is interposed between the receiver antenna 120 and the high-frequency mixer 122. The switching circuit 138 is cyclically activated and deactivated so as to selectively transmit the received signal Sr to the high-frequency mixer 122 in response to the switching signal supplied from the second oscillator 136. Thus, the high-frequency mixer 122 produces the beat signal Sb having a frequency controlled in accordance with the switching signal.
A band-pass filter 132 extracts frequency components of the beat signal Sb thus selected and produced from the high-frequency mixer 122. A band-pass filter 140 shapes the switching signal supplied from the second oscillator 136. An intermediate-frequency mixer 134 mixes the filtered signals supplied from the band-pass filters 132 and 140, and produces a converted beat signal Sb2 having a frequency in an inherent range of several tens to hundreds kHz.
According to the above-described radar system 110, it becomes possible to produce a beat signal having a frequency of several MHz which receives no substantial influence of the low-frequency noises when the switching signal has a frequency of several MHz. The intermediate-frequency mixer 134 handles the beat signal and the switching signals of several MHz. Thus, the frequency range handled by the intermediate-frequency mixer 134 is lower than the millimeter wave handed by the high-frequency mixer 122. The low-frequency noises involved in the output of the intermediate-frequency mixer 134 are smaller than those of the high-frequency mixer 122. The beat signal Sbs produced from the intermediate-frequency mixer 134 has an improved S/N ratio.
However, according to the above-described conventional radar system 110, the switching circuit 138 is interposed between the receiver antenna 120 and the high-frequency mixer 122. In other words, the switching circuit 138 is in a transmission path of the received signal Sr in a millimeter wave band. This is disadvantageous because the received signal Sr, significantly weakened and returned from the target object, is further attenuated, causing a serious deterioration in the sensitivity of the target object detection.
The switching circuit 138 is basically a high-frequency circuit processing millimeter wave band signals. Such a high-frequency circuit is difficult to install into a general circuit, expensive in cost, and time consuming in manufacturing. Thus, the price of the system will be increased significantly.